Most higher plants encounter at least transient decreases in relative water content at some stage of their life cycle and, as a result, have evolved a number of desiccation protection mechanisms. If however, the change in water deficit is prolonged the effects on the plants growth and development can be profound. Decreased water content due to drought, cold or salt stress can irreparably damage plant cells which in turn limits plant growth and crop productivity in agriculture.
Plants respond to adverse conditions of drought, salinity and cold with a variety of morphological and physiological changes. Although our understanding of plant tolerance mechanisms to these stresses is incomplete, the plant hormone abscisic acid (ABA) is believed to be an essential mediator between environmental stimulus and plant responses. ABA levels increase in response to water deficits and exogenously applied ABA mimics many of the responses induced by water-stress. Once ABA is synthesized it causes the closure of the leaf stomata thereby decreasing water loss through transpiration.
The identification of genes that transduce ABA into a cellular response opens the possibility of exploiting these regulators to enhance desiccation tolerance in crop species. In principle, these ABA signaling genes can be coupled with the appropriate controlling elements to allow optimal plant growth, development and productivity. Thus, not only would these genes allow the genetic tailoring of crops to withstand transitory environmental stresses, but they should also broaden the environments where traditional crops can be grown.
The recent isolation of an Arabidopsis thaliana mutant, era1, is hypersensitive to ABA and has been shown to also be tolerant to conditions of water deprivation. ERA1 has been identified as a β subunit of farnesyl transferase. Farnesyl transferase is a heterodimeric enzyme that provides the specific addition of a farnesyl pyrophosphate moiety onto the substrate target sequence. The target sequence is defined as a sequence of four amino acids which are present at the carboxy terminus of the protein and is referred to as a CaaX motif in which the “C” is cysteine, “a” is any aliphatic amino acid and “X” is any amino acid. The α subunit is common with a second prenylation enzyme, geranylgeranyl transferase, that has a different β subunit and adds a geranylgeranyl isoprenyl pyrophosphate moiety to the target sequence.
Prenylation is a multistep pathway which includes prenylation of the cysteine residue of the CaaX site, cleavage of the -aaX tripeptide and methylation of the prenyl-cysteine residue. Potentially, each of these steps could represent a target for genetic manipulation of the prenylation process to generate a desired phenotype such as stress tolerance.
In plants, prenylation has been linked to cell cycle control, meristem development, and phytohormone signal transduction, however, few details of the role of prenylation, the substrate proteins or the extent to which the plant system will be analogous to the mammalian and yeast systems are known. The most characterized substrates for CaaX modification are the Ras and a-factor proteins of yeast. Although there are three steps to complete protein maturation, abolition or modification of any one step does not necessarily result in cessation of target biological activities. Ras function is attenuated if the -aaX tripeptide is not cleaved but not abolished and some proteins retain the -aaX tripeptide after farnesylation. These observations may be substrate specific as, in contrast, there are examples indicating some proteins are fully functional only after being properly prenylated such as in regulating processes such as mitogen response in mammals and mating pheromone in yeast.
In Arabidopsis thaliana, more than 600 proteins contain a CaaX motif, suggesting a role for the post-translational modification by prenylation in numerous cellular processes. In Arabidopsis thaliana, it has been demonstrated that the loss-of-function of the β-subunit of farnesyl transferase will result in a ABA-hypersensitive phenotype. Although it is still not clear why plants lacking the functional β-subunit of farnesyl transferase become more sensitive to ABA, it clearly suggests that protein prenylation is involved in regulation of the homeostasis of ABA sensitivity. The balance of ABA cellular responses, whether more sensitive or less sensitive to ABA, is possibly regulated by the relative activities of prenylated proteins.
This invention is directed at the manipulation of the farnesyl transferase (FT) subunits, either α or β (FTA, FTB) to alter farnesyl transferase enzyme expression and activity. Farnesyl transferase catalyses the first step of farnesylation in which a 15-carbon farnesyl moiety is added to the cysteine residue of the target sequence CaaX. Included in this invention are vector constructs containing FTA or FTB sequences under the control of appropriate regulatory sequences to produce phenotypes such as, but not limited to, water-stress tolerance, increased biomass accumulation, increased yield or delayed senescence. Manipulation of the FTA subunit may also affect the activity of geranylgeranyl transferase and the phenoytypes associated with this manipulation are encompassed by this invention.